A week after confidently announcing its forthcoming season, English National Opera, still apparently a shaky edifice after a troubled recent history, has been hit by the resignation by a senior member of its management team.

Camilla Nicholls, the director of communications, has stepped down after just six months in the job. Yesterday she said the reason for her sudden resignation could be summed up in two words - "one beginning with S, and one beginning with D" - referring to the artistic director and chief executive of ENO, Sean Doran.

Ms Nicholls yesterday paid tribute to colleagues. "I think ENO is great and I played a tiny, tiny part in its history. I had some fantastic relationships with people. My team was just brilliant, the music director [Paul Daniel] is extraordinary. These are committed, loyal, lovely people creating magic for thousands night after night."

She declined to comment further. But according to a source who preferred not to be named, the breakdown in relations between Ms Nicholls and Mr Doran occurred during her six-month probationary interview on Wednesday. According to the source, she was blamed for recent lukewarm reviews and poor box office - "basically every single thing that was going wrong with ENO".

Mr Doran informed her, according to the source, that he wished to extend her probationary period by a further three months, but Ms Nicholls felt her position was untenable.

She resigned the following day and, according to the source, is considering suing for constructive dismissal.

A spokesman for the company denied that there had been a clash between Ms Nicholls and Mr Doran: "She has very good relationships within the company, and is very respected and admired. I'm not aware of any particular reason why she decided to resign."

In a message to staff, Mr Doran referred to Ms Nicholls's "visible and positive impact on behalf of ENO" and her contribution to "leading us towards a more stable and upbeat future".

ENO has suffered setbacks since the new year.

Its return to the refurbished Coliseum was delayed with the loss of the long-awaited revival of John Adams' Nixon in China, and Phyllida Lloyd's new production of Wagner's Ring cycle has received mixed reviews.